Papers by Evgenia(Zhenya) Ryabenko
Procedia Earth and Planetary Science, 2015
Over much of the ocean’s surface, productivity and growth are limited by a scarcity of bioavailab... more Over much of the ocean’s surface, productivity and growth are limited by a scarcity of bioavailable nitrogen. Sedimentary 15N
records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of
these changes has been hindered by the complexity of nitrogen isotope cycling. Herewe present a database of 15Nin sediments
throughout the world’s oceans, including 2,329 modern seafloor samples, and 76 timeseries spanning the past 30,000 years.
We show that the 15N values of modern seafloor sediments are consistent with values predicted by our knowledge of nitrogen
cycling in the water column. Despite many local deglacial changes, the globally averaged 15N values of sinking organic matter
were similar during the Last Glacial Maximum and Early Holocene. Considering the global isotopic mass balance, we explain
these observations with the following deglacial history of nitrogen inventory processes. During the Last Glacial Maximum,
the nitrogen cycle was near steady state. During the deglaciation, denitrification in the pelagic water column accelerated.
The flooding of continental shelves subsequently increased denitrification at the seafloor, and denitrification reached near
steady-state conditions again in the Early Holocene.We use a recent parameterization of seafloor denitrification to estimate a
30–120% increase in benthic denitrification between 15,000 and 8,000 years ago. Based on the similarity of globally averaged
15N values during the Last Glacial Maximum and Early Holocene, we infer that pelagic denitrification must have increased by
a similar amount between the two steady states.
[1] Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the p... more [1] Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.
Fixed nitrogen (N) loss to biogenic N 2 in intense oceanic O 2 minimum zones (OMZ) accounts for a... more Fixed nitrogen (N) loss to biogenic N 2 in intense oceanic O 2 minimum zones (OMZ) accounts for a large fraction of the global N sink and is an essential control on the ocean's N-budget. However, major uncertainties exist regarding microbial pathways as well as net impact on the magnitude of N-loss and the ocean's overall N-budget. Here we report the discovery of a N-loss hotspot in the Peru OMZ associated with a coastally trapped mesoscale eddy that is marked by an extreme N-deficit matched by biogenic N 2 production, high NO − 2 levels, and the highest isotope enrichments observed so far in OMZ's for the residual NO − 3 . High sea surface chlorophyll in seaward flowing streamers provides evidence for offshore eddy transport of highly productive, inshore water. Resulting pulses in the downward flux of particles likely stimulated heterotrophic dissimilatory NO − 3 reduction and subsequent production of biogenic N 2 within the OMZ. A shallower biogenic N 2 maximum within the oxycline is likely a feature advected by the eddy streamer from the shelf. Eddy-associated temporal-spatial heterogeneity of N-loss, mediated by a local succession of microbial processes, may explain inconsistencies observed among prior studies. Similar transient enhancements of N-loss likely occur within all other major OMZ's exerting a major influence on global ocean N and N isotope budgets.
We present new nitrogenisotope data from the water column and surface sediments for paleo-proxy v... more We present new nitrogenisotope data from the water column and surface sediments for paleo-proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the δ15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (δ15NNOx) in the upper 200 m of the water column. Between 1°N-10°S, subsurface O2 is still high enough to suppress N-loss keeping δ15NNOx values relatively low in the subsurface waters. However δ15NNOx values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. δ15Nsed is consistently lower than the isotopic signature of upwelled NO3−, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled NO3− are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface δ15NNOx values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north-south gradient towards stronger surface NO3− utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, δ15Nsed is lower than maximum water column δ15NNOx values most likely because only a portion of the upwelled water originates from the depths where highest δ15NNOx values prevail. Though the enrichment of δ15NNOx in the subsurface waters is unambiguously reflected in δ15Nsed values, the magnitude of δ15Nsed enrichment depends on both the depth of upwelled waters and high subsurface δ15NNOx values produce by N-loss. Overall, the degree of N-loss influencing subsurface δ15NNOx values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo δ15Nsed records from the Peruvian oxygen minimum zone.
We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasti... more We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting δ15N distributions. Pacific surface waters
are marked by strongly positive values for δ15N-NO3- reflecting fractionation associated with subsurface N-loss and partial NO3-utilization. This contrasts with negative values in NO3- depleted surface waters of the Atlantic which are lower than can be explained by N supply via N2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively δ15 low (d =11.4‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations [O2]>50 μmol l−1 in both regions reflect a nitrification source. Sharp decrease in N2O
concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 <5 μmol l−1.
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Papers by Evgenia(Zhenya) Ryabenko
records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of
these changes has been hindered by the complexity of nitrogen isotope cycling. Herewe present a database of 15Nin sediments
throughout the world’s oceans, including 2,329 modern seafloor samples, and 76 timeseries spanning the past 30,000 years.
We show that the 15N values of modern seafloor sediments are consistent with values predicted by our knowledge of nitrogen
cycling in the water column. Despite many local deglacial changes, the globally averaged 15N values of sinking organic matter
were similar during the Last Glacial Maximum and Early Holocene. Considering the global isotopic mass balance, we explain
these observations with the following deglacial history of nitrogen inventory processes. During the Last Glacial Maximum,
the nitrogen cycle was near steady state. During the deglaciation, denitrification in the pelagic water column accelerated.
The flooding of continental shelves subsequently increased denitrification at the seafloor, and denitrification reached near
steady-state conditions again in the Early Holocene.We use a recent parameterization of seafloor denitrification to estimate a
30–120% increase in benthic denitrification between 15,000 and 8,000 years ago. Based on the similarity of globally averaged
15N values during the Last Glacial Maximum and Early Holocene, we infer that pelagic denitrification must have increased by
a similar amount between the two steady states.
are marked by strongly positive values for δ15N-NO3- reflecting fractionation associated with subsurface N-loss and partial NO3-utilization. This contrasts with negative values in NO3- depleted surface waters of the Atlantic which are lower than can be explained by N supply via N2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively δ15 low (d =11.4‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations [O2]>50 μmol l−1 in both regions reflect a nitrification source. Sharp decrease in N2O
concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 <5 μmol l−1.
Books by Evgenia(Zhenya) Ryabenko
records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of
these changes has been hindered by the complexity of nitrogen isotope cycling. Herewe present a database of 15Nin sediments
throughout the world’s oceans, including 2,329 modern seafloor samples, and 76 timeseries spanning the past 30,000 years.
We show that the 15N values of modern seafloor sediments are consistent with values predicted by our knowledge of nitrogen
cycling in the water column. Despite many local deglacial changes, the globally averaged 15N values of sinking organic matter
were similar during the Last Glacial Maximum and Early Holocene. Considering the global isotopic mass balance, we explain
these observations with the following deglacial history of nitrogen inventory processes. During the Last Glacial Maximum,
the nitrogen cycle was near steady state. During the deglaciation, denitrification in the pelagic water column accelerated.
The flooding of continental shelves subsequently increased denitrification at the seafloor, and denitrification reached near
steady-state conditions again in the Early Holocene.We use a recent parameterization of seafloor denitrification to estimate a
30–120% increase in benthic denitrification between 15,000 and 8,000 years ago. Based on the similarity of globally averaged
15N values during the Last Glacial Maximum and Early Holocene, we infer that pelagic denitrification must have increased by
a similar amount between the two steady states.
are marked by strongly positive values for δ15N-NO3- reflecting fractionation associated with subsurface N-loss and partial NO3-utilization. This contrasts with negative values in NO3- depleted surface waters of the Atlantic which are lower than can be explained by N supply via N2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively δ15 low (d =11.4‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations [O2]>50 μmol l−1 in both regions reflect a nitrification source. Sharp decrease in N2O
concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 <5 μmol l−1.